The manifestation of Huntington's disease (HD) has been correlated with a CAG trinucleotide repeat expansion (encoding polyglutamine) in a gene (HD) residing on chromosome 4p16.3 that encodes a protein (huntingtin) of unknown function. One of our aims is to contribute information pertaining to the normal function of huntingtin from the results of ablation of the cognate gene (Hdh) in mice. For this purpose, we have disrupted the Hdh gene in embryonic stem (ES) cells and obtained extensively chimeric mice by injecting targeted clones into host blastocysts. When chimeras transmitting the mutation are identified, we propose to initiate a breeding program between heterozygous offspring, to obtain homozygous mutants and study in detail their null phenotype. A second aim that will be pursued in parallel to complement the first will be the development of a mouse model hopefully simulating HD. This will be accomplished using again a gene targeting strategy, by replacing a region of the Hdh gene with the homologous segment of a human HD gene cloned from the DNA of a juvenile HD patient, which contains highly expanded CAG triplets (94 vs. 7 in Hdh). The phenotypic consequences of this replacement mutation in mice, which should exhibit dominance, are expected to be different than those in null mutants; according to our working hypothesis, the mutant huntingtin, containing an expanded polyglutamine stretch, maintains activity through a normal pathway, but also acquires novel, abnormal interactions with other cellular effectors. Heterozygous and homozygous mice carrying this "dominant" mutation will be studied extensively for phenotypic manifestations, using a variety of tests that can assess movement, cognitive and behavioral abnormalities, and also by detailed histopathological and immunochemical analyses. This animal model should be valuable for a variety of studies, including the potential testing of therapeutic regimes. The genetic program that we propose should allow the establishment of causal relationships between defined mutations and their phenotypic consequences, and has the advantage that questions pertinent to a human disease will be addressed in vivo and in the context of the entire developing model organism.